Sewer manholes are openings through the street surface to a sewer to provide easier inspection and cleaning. They are placed usually at intervals of 300 to 500 feet and a straight line portion of the sewer extends between a pair of adjacent manholes. Manholes are usually circular in plan, having a cast-iron opening in the street surface which is covered with a corrugated cast-iron cover. Usually the manhole enlarges to about 4 feet in diameter and continues at this diameter down to the sewer elevation. This provides ample space for cleaning or unstopping the sewer with the conventional segmented sewer rods commonly used. The sewage is carried through the manhole floor in channels in the floor with the depth of the channels from 1/2 of the sewer diameter up to the full diameter of the sewer ends which extend into the manhole.
In some constructions sewer manholes are poured concrete and in other constructions and as shown in the preferred embodiment consist of a series of cylindrical reinforced concrete sections having cooperating annular portions and grooves to provide an upright cylinder and including a cylindrical bottom section which includes bottom and circular side walls.
Heretofore, to provide a flooring for the bottom section of the manhole which is spaced below sewer conduits a quantity of concrete is normally poured which extends from the bottom of the section up to and adjacent the side wall openings which receive the sewer conduits. Conventionally a U-shaped channel is formed in the top surface of the flooring which communicates with respective inlet and outlet or a plurality of inlets and outlets so that sewage from the sewer conduits flows through the channels in the bottom of the flooring directly through the manhole construction and outwardly thereof.
Examples of manhole constructions are shown in the following U.S. Pat. Nos.: 4,102,088; 4,127,990; 2,166,632; 3,276,176.
These patents are identified merely as showing the various types of sewer manhole constructions.